This invention relates to a spark plug including a center electrode and a ground electrode disposed in a confronting relationship with a noble metallic firing tip fixed on an opposed portion of at least one of these electrodes.
A conventional spark plug includes a noble metallic firing tip, for example made of a Pt (platinum) alloy or an Ir (iridium) alloy, having excellent spark exhaustion resistance as a spark discharge electrode member disposed in a discharge gap defined between a center electrode and a ground electrode.
The spark plug employing a noble metallic firing tip electrode is advantageous in that the radial size of the electrode can be reduced. Furthermore, thinning the electrode brings the effect of reducing flame quenching function and accordingly assures higher ignitability.
In general, the growth of flame kernel formed during spark discharge determines the ignitability. The electrode, being a large heat mass, tends to cool the flame kernel and accordingly disturbs the growth of flame kernel. In this respect, the slender noble metallic firing tip electrode having a thinned diameter brings desirable effect of suppressing the flame kernel cooling function of the electrode.
However, according to the conventional spark plug, the flame kernel does not grow from the same position. For example, the flame kernel may grow from a portion of the discharge surface being offset toward the leg of the ground electrode or, in alternative, from an opposite portion being offset toward a distal end of the ground electrode.
When the flame kernel is formed at the distal end side of the ground electrode, the growth of flame kernel is relatively smooth and easy because the flame kernel is not so severely subjected to the above-described cooling function of the ground electrode. On the other hand, when the flame kernel is formed at the opposite side offset toward the leg (i.e., a proximal end) of the ground electrode, the growth of flame kernel is relatively difficult because the flame kernel is directly subjected to the above-described cooling function of the ground electrode (including its leg portion).
In general, the ignitability of a spark plug is improved when the discharge gap is widened. In other words, a brand-new spark plug has the worst ignitability. Similar tendency is recognized even in a spark plug employing a noble metallic firing tip electrode. Furthermore, this tendency is remarkably recognized when an initial setting of the discharge gap is short.